--
-- json.lua
--
-- Copyright (c) 2020 rxi
--
-- Permission is hereby granted, free of charge, to any person obtaining a copy of
-- this software and associated documentation files (the "Software"), to deal in
-- the Software without restriction, including without limitation the rights to
-- use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
-- of the Software, and to permit persons to whom the Software is furnished to do
-- so, subject to the following conditions:
--
-- The above copyright notice and this permission notice shall be included in all
-- copies or substantial portions of the Software.
--
-- THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-- IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-- FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-- AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-- LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-- OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-- SOFTWARE.
--
json = {
    _version = '0.1.2'
}

-------------------------------------------------------------------------------
-- Encode
-------------------------------------------------------------------------------

local encode

local escape_char_map = {
    ['\\'] = '\\',
    ['"'] = '"',
    ['\b'] = 'b',
    ['\f'] = 'f',
    ['\n'] = 'n',
    ['\r'] = 'r',
    ['\t'] = 't'
}

local escape_char_map_inv = {
    ['/'] = '/'
}
for k, v in pairs(escape_char_map) do
    escape_char_map_inv[v] = k
end

local function escape_char(c)
    return '\\' .. (escape_char_map[c] or string.format('u%04x', c:byte()))
end

local function encode_nil(val)
    return 'null'
end

local function encode_table(val, stack)
    local res = {}
    stack = stack or {}

    -- Circular reference?
    if stack[val] then
        error('circular reference')
    end

    stack[val] = true

    if rawget(val, 1) ~= nil or next(val) == nil then
        -- Treat as array -- check keys are valid and it is not sparse
        local n = 0
        for k in pairs(val) do
            if type(k) ~= 'number' then
                error('invalid table: mixed or invalid key types')
            end
            n = n + 1
        end
        if n ~= #val then
            error('invalid table: sparse array')
        end
        -- Encode
        for i, v in ipairs(val) do
            table.insert(res, encode(v, stack))
        end
        stack[val] = nil
        return '[' .. table.concat(res, ',') .. ']'

    else
        -- Treat as an object
        for k, v in pairs(val) do
            if type(k) ~= 'string' then
                error('invalid table: mixed or invalid key types')
            end
            table.insert(res, encode(k, stack) .. ':' .. encode(v, stack))
        end
        stack[val] = nil
        return '{' .. table.concat(res, ',') .. '}'
    end
end

local function encode_string(val)
    return '"' .. val:gsub('[%z\1-\31\\"]', escape_char) .. '"'
end

local function encode_number(val)
    -- Check for NaN, -inf and inf
    if val ~= val or val <= -math.huge or val >= math.huge then
        error('unexpected number value \'' .. tostring(val) .. '\'')
    end
    return string.format('%.14g', val)
end

local type_func_map = {
    ['nil'] = encode_nil,
    ['table'] = encode_table,
    ['string'] = encode_string,
    ['number'] = encode_number,
    ['boolean'] = tostring
}

encode = function(val, stack)
    local t = type(val)
    local f = type_func_map[t]
    if f then
        return f(val, stack)
    end
    error('unexpected type \'' .. t .. '\'')
end

function json.encode(val)
    return (encode(val))
end

-------------------------------------------------------------------------------
-- Decode
-------------------------------------------------------------------------------

local parse

local function create_set(...)
    local res = {}
    for i = 1, select('#', ...) do
        res[select(i, ...)] = true
    end
    return res
end

local space_chars = create_set(' ', '\t', '\r', '\n')
local delim_chars = create_set(' ', '\t', '\r', '\n', ']', '}', ',')
local escape_chars = create_set('\\', '/', '"', 'b', 'f', 'n', 'r', 't', 'u')
local literals = create_set('true', 'false', 'null')

local literal_map = {
    ['true'] = true,
    ['false'] = false,
    ['null'] = nil
}

local function next_char(str, idx, set, negate)
    for i = idx, #str do
        if set[str:sub(i, i)] ~= negate then
            return i
        end
    end
    return #str + 1
end

local function decode_error(str, idx, msg)
    local line_count = 1
    local col_count = 1
    for i = 1, idx - 1 do
        col_count = col_count + 1
        if str:sub(i, i) == '\n' then
            line_count = line_count + 1
            col_count = 1
        end
    end
    error(string.format('%s at line %d col %d', msg, line_count, col_count))
end

local function codepoint_to_utf8(n)
    -- http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=iws-appendixa
    local f = math.floor
    if n <= 0x7f then
        return string.char(n)
    elseif n <= 0x7ff then
        return string.char(f(n / 64) + 192, n % 64 + 128)
    elseif n <= 0xffff then
        return string.char(f(n / 4096) + 224, f(n % 4096 / 64) + 128, n % 64 + 128)
    elseif n <= 0x10ffff then
        return string.char(f(n / 262144) + 240, f(n % 262144 / 4096) + 128, f(n % 4096 / 64) + 128, n % 64 + 128)
    end
    error(string.format('invalid unicode codepoint \'%x\'', n))
end

local function parse_unicode_escape(s)
    local n1 = tonumber(s:sub(1, 4), 16)
    local n2 = tonumber(s:sub(7, 10), 16)
    -- Surrogate pair?
    if n2 then
        return codepoint_to_utf8((n1 - 0xd800) * 0x400 + (n2 - 0xdc00) + 0x10000)
    else
        return codepoint_to_utf8(n1)
    end
end

local function parse_string(str, i)
    local res = ''
    local j = i + 1
    local k = j

    while j <= #str do
        local x = str:byte(j)

        if x < 32 then
            decode_error(str, j, 'control character in string')

        elseif x == 92 then -- `\`: Escape
            res = res .. str:sub(k, j - 1)
            j = j + 1
            local c = str:sub(j, j)
            if c == 'u' then
                local hex = str:match('^[dD][89aAbB]%x%x\\u%x%x%x%x', j + 1) or str:match('^%x%x%x%x', j + 1) or
                                decode_error(str, j - 1, 'invalid unicode escape in string')
                res = res .. parse_unicode_escape(hex)
                j = j + #hex
            else
                if not escape_chars[c] then
                    decode_error(str, j - 1, 'invalid escape char \'' .. c .. '\' in string')
                end
                res = res .. escape_char_map_inv[c]
            end
            k = j + 1

        elseif x == 34 then -- `"`: End of string
            res = res .. str:sub(k, j - 1)
            return res, j + 1
        end

        j = j + 1
    end

    decode_error(str, i, 'expected closing quote for string')
end

local function parse_number(str, i)
    local x = next_char(str, i, delim_chars)
    local s = str:sub(i, x - 1)
    local n = tonumber(s)
    if not n then
        decode_error(str, i, 'invalid number \'' .. s .. '\'')
    end
    return n, x
end

local function parse_literal(str, i)
    local x = next_char(str, i, delim_chars)
    local word = str:sub(i, x - 1)
    if not literals[word] then
        decode_error(str, i, 'invalid literal \'' .. word .. '\'')
    end
    return literal_map[word], x
end

local function parse_array(str, i)
    local res = {}
    local n = 1
    i = i + 1
    while 1 do
        local x
        i = next_char(str, i, space_chars, true)
        -- Empty / end of array?
        if str:sub(i, i) == ']' then
            i = i + 1
            break
        end
        -- Read token
        x, i = parse(str, i)
        res[n] = x
        n = n + 1
        -- Next token
        i = next_char(str, i, space_chars, true)
        local chr = str:sub(i, i)
        i = i + 1
        if chr == ']' then
            break
        end
        if chr ~= ',' then
            decode_error(str, i, 'expected \']\' or \',\'')
        end
    end
    return res, i
end

local function parse_object(str, i)
    local res = {}
    i = i + 1
    while 1 do
        local key, val
        i = next_char(str, i, space_chars, true)
        -- Empty / end of object?
        if str:sub(i, i) == '}' then
            i = i + 1
            break
        end
        -- Read key
        if str:sub(i, i) ~= '"' then
            decode_error(str, i, 'expected string for key')
        end
        key, i = parse(str, i)
        -- Read ':' delimiter
        i = next_char(str, i, space_chars, true)
        if str:sub(i, i) ~= ':' then
            decode_error(str, i, 'expected \':\' after key')
        end
        i = next_char(str, i + 1, space_chars, true)
        -- Read value
        val, i = parse(str, i)
        -- Set
        res[key] = val
        -- Next token
        i = next_char(str, i, space_chars, true)
        local chr = str:sub(i, i)
        i = i + 1
        if chr == '}' then
            break
        end
        if chr ~= ',' then
            decode_error(str, i, 'expected \'}\' or \',\'')
        end
    end
    return res, i
end

local char_func_map = {
    ['"'] = parse_string,
    ['0'] = parse_number,
    ['1'] = parse_number,
    ['2'] = parse_number,
    ['3'] = parse_number,
    ['4'] = parse_number,
    ['5'] = parse_number,
    ['6'] = parse_number,
    ['7'] = parse_number,
    ['8'] = parse_number,
    ['9'] = parse_number,
    ['-'] = parse_number,
    ['t'] = parse_literal,
    ['f'] = parse_literal,
    ['n'] = parse_literal,
    ['['] = parse_array,
    ['{'] = parse_object
}

parse = function(str, idx)
    local chr = str:sub(idx, idx)
    local f = char_func_map[chr]
    if f then
        return f(str, idx)
    end
    decode_error(str, idx, 'unexpected character \'' .. chr .. '\'')
end

function json.decode(str)
    if type(str) ~= 'string' then
        error('expected argument of type string, got ' .. type(str))
    end
    local res, idx = parse(str, next_char(str, 1, space_chars, true))
    idx = next_char(str, idx, space_chars, true)
    if idx <= #str then
        decode_error(str, idx, 'trailing garbage')
    end
    return res
end

return json
